Well, an SX-980 is next to the top of the line. 1080 was the top.
The G-7700 is two down from the top. G-8700 and G-9700 above it.
He's asking you to add money to your 980 to get his 7700. Right?
I'm not sure I'd even do an even trade. If the conditions of the two receivers makes it work, but like I said, I think it's rude of him to ask for money with your receiver.