Question: I loaned someone money which would be paid back after 3 years. Do I have to pay Zakaat on it?
Answer: Zakat on the loan you forwarded would not be payable now until you take possession of it. Once it comes back into your possession, Zakaat would be compulsory for all the previous years as well.
And Almighty Allah knows best
Fatāwā Dārul Ulūm Zakariyyā, Vol. 3, Pg. 122-123