The builders have already damaged the carpet! If the adhesive (probably gaffa tape ::) ) has left a residue it more than likely took away a load of fibre from the pile when it was yanked up.
I'd take this on under a disclaimer that the most appropriate solvent to remove the glue may also permanently damage the capet. I wouldn't issue any guarantees as to the likely success either. This is a "salvage attempt". As long as you're straight with them and don't put them under any false illusions, you'll be fine.
! ! ! GET A WRITTEN QUOTATION SIGNED BEFORE YOU DO ANYTHING ! ! !
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Rinse through with plain hot water first, then try a few different solvents by applying them indirectly via a nappy towel and blotting, not rubbing.
You should see the effectiveness by the transfer to the nappy towel. Once you find the right product for the job, carry on using the towel and allow a bit more dwell time before rinsing out. You may need to do it twice.
Don't rip yourself off price-wise. Don't do it at a knock-down price hoping it will curry favour for more work either, the building company won't reciprocate as a general rule.