June 2011 Eng. Law Q8

Q.8 Part B relates to the waiver of existing contractual rights and in particular the part payment by a third party.
My question is that why a part payment by a third party is acceptable? Does it not give rise to people using third parties to pay only partial amounts of their debts?
For example party A owes party G $1000, but party A uses party X as a third party and pays party G only $800 and the poor party G cannot even sue them?? Doesnt make much sense!!