I think the Sansui is a better receiver and more worth?
If it's not a fair deal, how much money should I ask (or give) on top of the trade?
Yes, I'd say the G7000 is worth a little more in comparable condition.
The G7000 and SX980 had similar power, i.e. 85 watts per channel, but the original price was a little higher on the Sansui, $750 vs. $650 for the Pioneer ... see classicaudio.com, here.
On top of that, Sansui equipment is reputedly a warmer, fuller sound vs. the Pioneer, and many vintage user prefer it for that reason. If you can listen to both units in the same setting, you can make your own judgement of course.
In the first place I wouldn't trade the Sansui for the Pioneer at all, but if you do, perhaps ask for $100 cash along with the unit.
Sansui may be warmer, but the Pioneer has better stereo separation and clarity, IMO. Now on the other hand, the earlier XX50 series Pioneers are warmer like the Sansui's. Now all of them are great in their own way, you just have to see which one sounds better to your ears. Just wish the Sansui's woulda put real walnut veneer on their cases instead of that vinyl junk.
If everybody had everything they wanted, the world would be a pretty boring place :)