Personally, I wouldn't give in to the demand of the contractor. I had a case where the contractor was supposed to do up three floors of buildings for me. The schedule payment follows and suddenly there is a price increase in material (prior to Beijing 2008) they demand extra. They are already late in delivery and if they had delivered earlier according to schedule, then they would have made enough to cover the price increase. They went on strike and I got another group in to finish up the work for a renegotiated price. I don't mind paying extra but will not pay for people's faults and problems.