Was it just the sender's cheque that was incorrect or were they incorrectly invoiced in the first place? The part which says "... the two invoices of Mr Leong ($100 and $50)" suggests that that is what they were told the costs were.
If they were invoiced for a total of $150 and that is what they have paid, you would need to add an apology to the letter, something like "Thank you for your cheque for $150 in payment for Mr Leong's invoices. Unfortunately, the original invoices were incorrect and should have been for $200 and $50 respectively, making a total of $250. The new invoices are attached to this letter/email. We are returning your cheque. Please submit a cheque for $250. Thank you."
If they were invoiced for $250 and have inexplicably sent a cheque for only $150, the letter/email would look more like this. "Thank you for your cheque for $150 in payment for Mr Leong's invoices. However, the invoices were for $200 and $50 respectively (copies attached). We are returning your cheque. Please submit a cheque for the full $250 as soon as possible".
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