Chapter 46 of the Ganapati Khanda of Brahma-Vaivarta Purana gives an account of the conversation between Ganesha and Tulsi.
It starts with Narada asking Narayana why Tulsi leaves are not used in worship of Ganesha.
पूजां भगवतश्चक्रे रामो गणपतेर्यदा। नैवेद्यैर्विविधैः पुष्पैस्तुलसीं च विना कथम्॥४॥
तुलसी सर्वपुष्पाणां मान्या धन्या मनोहरा। कथं पूतां सारभूतां न गृह्णाति गणेश्वरः।।५।।
Narada said - Parasurama offered puja to Ganesa with all the eatables,
flowers, etc., but how could his adoration be accepted without the
Tulasi leaves because of all the flowers, Tulasi is considered to be
the best; then why Tulasi leaves are not acceptable by Ganesa?
शृणु नारद वक्ष्येऽहमितिहासं पुरातनम्। ब्रहाकल्पस्य वृतान्तं निगूढं च मनोहरम्।६।
Narayana said - O Narada, I am going to tell you an ancient story
which is contained as an ancient Secret. You listen to it.
Narayana recites a story where Tulsi meets Ganesha who is engaged in Tapas at the banks of the Ganga. Feeling attracted to the handsome Ganesha Tulsi disturbs his Tapas.
एकदा तुलसी देवी प्रोद्भिन्ननवयौवना। तीर्थ भ्रमन्ती तपसा नारायणपरायणा।७।
ददर्श गङ्गातीरे सा गणेशं यौवनान्वितम्। अतीव सुन्दरं शुद्धं सस्मितं पीतवाससम्॥८॥
Once Tulasi in her youthful age, performing tapas and reciting the name of > Narayana was visiting holy places. Thereafter the youthful
Ganesa spotted her on the bank of the Ganga. He was quite beautiful,
having a spotless mind, wearing a serene smile on his face and was
clad in a yellow lower garment.
गणेशस्य प्रधानाङ्गे दत्त्वा किचिज्जलं मुने। जघान तर्जन्यग्रेण निष्पन्दं कृष्णमानसम्॥ १४।।
Then she dropped some water on Ganesa and pushed him with the tip of
her forefinger. Ganesa was engrossed in the devotion of lord Krsna.
बभूव ध्यानभग्नं च तस्य नारद चेतनम्। दुःखं च ध्यानभेदेन तद्विच्छेदो हि शोकद:॥ १५॥
O Narada, this disturbed his meditation and he felt painful because
being disturbed in one's meditation is quite painful.
Ganesha awakes from his meditation to see Tulsi before him he enquires of her lineage and purpose. Tulsi introduces herself and proposes Ganesha for marriage.
ध्यानं त्यक्त्वा हरेिं स्मृत्वा चापश्यत्कामिनीं पुर:। नवयौवनसंपन्नां सस्मितां कामपीडिताम्॥ १६॥
Having been disturbed in meditation on Hari, he opened his eyes and
found a beautiful damsel standing before him who was quite youthful
and was smiling besides being infatuated with passion.
गणेश्वर उवाच का त्वं वत्से कस्य कन्या मातर्मा बूहि कि शुभे।
पापदोऽशुभदः शश्वद्द्ध्यानभङ्गस्तपस्विनाम्॥ १८॥
कृष्णः करोतु कल्याणं हन्तु विघ्नं कृपानिधिः। तद्धयानभङ्गजाहोषान्नशुर्भ स्यातु ते शुभे। १९॥
Ganesvara said - O damsel, who are you? Who is your father? O mother, O auspicious one, you tell me. It is sinful to disturb a
person performing the tapas which results in evil. O Virtuous one, let
lord Krsna bestow welfare on you and also remove all obstructions. Let
you be free of the sin of disturbing my meditation.
गणेशवचनं श्रुत्वा तमुवाच स्मरातुरा। सस्मितं सकटाक्षं च देवं मधुरया गिरा॥ २०॥
Oh hearing the words of Ganesa, the passionate Tulasi laughed and
casting side glances she spoke in a Sweet voice.
तुलस्युवाच धर्मात्मजस्य कन्याऽहमप्रौढा च तपस्विनी। तपस्या मे स्वामिनोऽर्थ त्वं स्वामी भव मे प्रभो। २१॥
Tulasi said - "O lord, I am the daughter of Dharmaputra. I am youthful and am an ascetic, I am performing tapas for achieving a
husband; therefore you be my husband."
Ganesha politely dismisses her proposal as he is not interested in getting married and asks her to look for another husband.
Tulsi feeling hurt by this rejection curses Ganesha that he would get a wicked wife. Ganesha counter-curses Tulsi that she would have to marry a demon and become a plant.
गणेश उवाच हे मातर्नास्ति मे वाञ्छा घोरे दारपरिग्रहे। दारग्रहो ही दुःखाय न सुखाय कदाचन॥२३॥
Ganesa said - O mother, it is terrible for me to accept a spouse. I
am, therefore not interested in the same. After marriage no one gets
pleasure but one is sure to get pain.
निवर्तस्व महाभागे पश्यान्यं कामुकं पतिम्। कामुकेनैव कामुक्याः संगमो गुणवान्भवेत्॥२७॥
Therefore, O virtuous one, you return and search for another
passionate husband. Because only a passionate one can be the husband
of a passionate lady and their union is always beneficial."
इत्येवं वचनं श्रुत्वा कोपात्सा तं शशाप ह। दारास्ते भविताऽसाध्वी गणेश्वर न संशयः॥ २८॥
इत्याकर्ण्य सुरश्रेष्ठस्तां शशाप शिवात्मजः।देवि त्वमसुरग्रस्ता भविष्यसि न संशयः।। २९॥
On hearing these words she pronounced a curse on him saying, "O
Ganesvara, you will get a wicked woman, there is no doubt about it."
On hearing this Ganesa also pronounced a curse on her saying, "O
goddess, you will be over-powered by a demon. There is no doubt about
तत्पश्चान्महतां शापाद्वृक्षस्त्वं भवितेति च। महातपस्वीत्युक्त्वा तां विरराम च नारद। ३०॥
Thereafter with the curse of elders you will have to be turned into
the form of a tree." Thus speaking the immensely intelligent Ganesa
Tulsi repents for her actions and offers prayers to Ganesha. Ganesha becoming pleased grants her boons that she would always be dear to Narayana but that he (Ganesha) would never accept her leaves.
शापं श्रुत्वा तु तुलसी सा रुरोद पुनः पुनः। तुष्टाव च सुरश्रेष्ठं स प्रसन्न उवाच ताम्॥ ३१॥
On hearing the curse Tulasi weeping again and again, started offering
prayer to lord Ganesa. Lord Ganesa was pleased with her and said.
गणेश्वर उवाच पुष्पाणां सारभूता त्वं भविष्यसि मनोरमे। कलांशेन महाभागे
Ganesvara said - O pleasant one, you will be the best of flowers known
as Tulasi, O virtuous one, because of the rays of lord Visnu, you will
become the beloved of Narayana.
प्रिया त्वं सर्वदेवानां श्रीकृष्णस्य विशेषतः॥ पूता विमुक्तिदा नृणां
मया भोग्या न नित्यशः॥३३॥
You will be loved by all and particularly you would be beloved of lord
Krsna, you will always remain auspicious and shall bestow salvation on
the people but I shall never accept your leaves.
पेन गणेशस्य च नारद। सा प्रिया शडुचूडस्य बभूव सुचिरं मुने॥३६॥
O Narada, O sage, because of the curse of Ganesa, the best of the
sages, Tulasi remained the wife of Sankhacuda for a long time.