oral agreement and refund

Once you both received money then it is your liable and duty to repay with genuine reasons. or otherwise F have right to initiate any legal actions as per the law

It depends.

Y may not be wantonly not returning the money and may not be in a position to refund when a sudden request is made from X withdrawing from the oral/mail partnership.

Therefore, if the request to refund is made orally, Y can very well explain the position orally. If X insists on Y to to refund in writing, let X give full details in such request for what purpose he gave the money. Then, Y can reply giving a brief background of their earlier oral/mail agreement and the purpose of such partnership. And also explain what Y did with that money with verifiable documentary proof and give valid and genuine reasons as to why he cannot refund it at this point of time. The language should be such that under no circumstances it can be imputed that  X paying to Y was as lender and borrower. Clearly the matter should be dealt with stating the Partnership and purpose of such partnership. Better professional help is taken from a capable and reliable Lawyer.

All this provided Y is genuine and honest. If he had influenced X in the name of ostensible partnership but swallowed the money thereby deceiving X he shall be in trouble as it's a criminal offence if proved.

Any activity connected with money and/or business should be in writing even if it's between close relatives like father and son, wife and husband or brothers.