The OP does not have the initial burden to prove anything. The person wanting the money back must first provide evidence that the money was some kind of loan rather than a gift since that person bears the burden of proof should the matter go to court. Now, if it was a loan of a “large sum of money” (and we do not know how much is involved here) one would expect that the donor would get a promissory note or other written evidence of the loan detailing the commitment to repay that is signed by the debtor. Even if it was an oral loan, they would need to have discussed when the loan would be repaid, what interest would be charged, etc. The lender would need to have some proof of that verbal contract. The lender’s own testimony would be admissible, but as it is self-serving might not get him very far. Since it is the lender that must prove the loan, the lack of a writing or other evidence may be a real problem for the supposed lender in this instance.
On the other hand, it is not very common to get signed written confirmation of gifts. So the fact that the OP doesn’t have anything in writing to prove the gift isn’t all that surprising. What else, besides his own testimony, would a jury expect him to have to prove a gift? And the supposed debtor only needs to come up with something on this if the lender offers some kind of evidence first that there was a loan.
If there is some other basis for the person asking for the money back, like a claim under a fraudulent conveyance statute, the person needs to make clear the basis for requesting the money back.

