Edit: this is not some stupid game or scam. There is a very serious reason for this research. And you will eventually come to know the reason. Please do help me ascertain the market rates for the above loan.
Yes it is. a NLockTime transaction is not valid until the time/block set by the transaction. Until that time/block, it would be trivial to double spend the transaction. The only way to protect against this is to fund a transaction to a 2-of-2 multisig address and then sign a NLockTime transaction from that multisig address to the lender's address.
There is also the risk that there will be some kind of fork (hard or soft) that would invalidate the NLockTime transaction that would prevent it from
ever confirming.
Lastly, what you are claiming to propose is that the lender sent you BTC and you subsequently return the BTC to the lender immediately with interest (at least from your point of view).