Can I "sell" Elgg?

I offer to build a community site for a company for a small fee. But they come with their lawyer who ask about "who own software". They are old fashion people. They want me sell them also software elgg. How can I sell something not mine is?

My first try to make money from elgg and I hit a lawyer....

I read Elgg licence GPL2 and MIT. I think I can "sell" them elgg. Only I must give them sourcecode with GPL2 and MIT licence explanations. Am I at fault? But the plugins? I dont know. Anybody has "sold" elgg?