gst payment or service tax payment

Hi All, I'm having some issues with transitional provisions. Section 142(11)(b) states that no tax shall be payable on services under this act to the extent tax was leviable on the said services under chapter V of fin act 1994

Certain st assessees like CA, Architects etc. with turnover less than 50 lakhs can pay service tax when they receive money from customers ignoring the pot rules.
A client of mine has received recovered from a debtor today and more is excepted in next month or so. Whole amount received is for invoices raised last year with service tax.
Based on this, how am I to deal with money received today and that which he'll receive next month
Thanks