1) you can obtain letter from aunt and her husband that in case payment of X amount is paid by you they would return money by september end
) it need not be on stamp paper
3) you cna accept post dated cheque for said amount
4) please note that in case cheque is dishonoured you would have to file complaint under section 138 NI
5) cheque bouncing cases take 5 years to be disposed of
6) you can also file summary suit to recover the money due and payable