What is the amount of zakat due on money that a partner returned after three years, given that he did not develop the agreed-upon funds and did not pay the profits?

This is a corrupt partnership, as it does not fall under either capital or credit partnerships. The earnings generated are the growth of one of the partners' assets. Therefore, you have the right to take all the profits accrued from your worker, and pay him a three-year wage for his work, estimated by experts. If you can prove that he concealed profits, you have the right to sue him in the Sharia courts. If you cannot do so, then there is no harm in retaining what he is due as wages in return for his injustice. You are obligated to pay Zakat for the three years at a rate of one-quarter of one-tenth (2.5%), unless the money was livestock at certain times, in which case livestock Zakat is paid for that period and cash Zakat for the remaining period.