It's just Vishnu's Lila.. You can see how farcical the situation is in this excerpt from the Sthala Purana (temple scripture) of Tirupati. Vishnu pretended to be so concerned about money that he didn't even want to give food to a messenger. Then he asked for a loan for wedding expenses from Kubera, who said that Vishnu is the supreme lord and all the Universe's riches belong to him, but agreed to give the loan anyway:
He thought that a messenger should leave in advance without taking food even. "My dear boy, proceed to Narayanapura in advance. Don't waste time. The four forces and sages will accompany you" said Srinivasa to a messenger. Brahma, however, felt that it would be better if the messenger left after taking food. "You are not considering the circumstances. You are speaking like a child. So much expenditure has to be met!" pointed out Srinivasan. Brahma kept quiet. Lord Shiva intervened and said: "As in the case of construction of a house, so also performance of marriage! One should try his best. Meeting heavily expenditure also, all arrangements have to be made." The bridgegroom asked "who will be my creditor to finance a grand marriage? Mere talk is of no use."
Kubera was present there. The bridegroom, Brahma and Shiva took him aside to a solitary place west of Swami Pushkarini. "Kindly advance the amount needed for the marriage to he bride-groom," Kubera requested. "The entire Universe is at your beck and call. All my money is yours. I have no control over it. You are its master" said Kubera [to Vishnu]. Finally, Kubera agreed to lend the money required if the necessary document was executed by the debtor. Brahma then dictated a promissory note. The note was executed. "This, the 7th day of the bright-fortnight of the month of Vysakha in Kaliyuga, The debtor is Srinivasa, the creditor is Dhaneswara. Purpose: marriage of the loanee. Amount of loan fourteen lakhs in Ramamudra coins. The loanee should repay it with interest in one thousand years after the loanee's marriage." Brahma and Siva attested as witnesses. The scribe was the loanee himself. After accepting the promissory note, Kubera paid the amount in cash.
I think that the 1000 years is Deva years, not human years, i.e. the duration of the loan is the Kali Yuga. In any case, this whole Lila of the loan benefits devotees, so that they can donate money to Venkateshwara to help repay the loan and thereby acquire Punya themselves.
