Why do you say yes if you need 600 euros to play and want to play for 45 minutes?’

The conditions say in the first line. what you have accepted are 20 min and 100 euros. No, you should not haggle offers afterward. 

It’s like that record deal you did early in your career, no you should not start to renegotiate the deal because something has changed. Sorry here is that you have to take a chance and you have to make the best of the situation. 

Start haggling and your reputation are out the window. 

My friend owned a club in a small city in 2000:s. He found a band online that he thought was really good and he managed to call them up and booked a gig with them 6 months ahead for like 100 euros. After two months the band got a smash hit and became the hottest band in Sweden. Everybody wanted them and their price went up to 2000 euros overnight.

Did they cancel the gig on my friend? No! Did they haggle the price? No! And that got them last night a new gig years after since I know that band stands for their values.