Manu Smriti 10.129
"No collection of wealth must be made by a Sudra, even though he be able (to do it); for a Sudra who has acquired wealth, gives pain to Brahmanas."
Here's the explanation given by Medhatithi in his commentary on the Manu Smriti:
‘Even though he is able’—by means of agriculture and such acts,—wealth shall not be amassed by the Śūdra. In support of this the Author adds an argument in the form of a declamatory statement—‘Having acquired wealth the Śūdra harasses the Brāhmaṇas.’ “What is the harassment caused to Brāhmaṇas?” Becoming very rich, they would make the Brāhmaṇas accept gifts from themselves, and the accepting of gifts from the Śūdra has been forbidden for them; hence becoming a party to their doing what is forbidden, he would incur sin. The danger of incurring such sin however could not apply to the case of one who goes on fulfilling all that is prescribed for him. Hence the ‘harassment’ of the Brāhmaṇa that is meant is only this that he would no longer serve them.
So the problem is that by acquiring wealth he may be tempted to stop performing his duty of serving Brahmanas and other members of the first three castes. And so he's forbidden from acquiring wealth in order to prevent this temptation.
However, if a low-caste person violates this and acquires wealth anyway, he should use this wealth to support members of the first three castes, as described in this chapter of the Gautama Dharma Sutras:
And a man of higher caste (who is his master and has fallen into distress must be maintained) by him. His hoard shall serve this purpose.