A credit entry of Rs450 on X's account in the books of Y could have arisen by ?

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It could be services randered by X amd amount is still payable. It is also possible x has overpaid to Y which need to be refund to X.

Y has created a credit entry in X's Account.

If you are speaking in terms of debtors, then the credit entry could have arisen due to payment by to Y via direct bank deposit, sales returns and also discounts.