Issuance of tax invoice

Under composition scheme dealer is not entitled to collect the amount of tax from customers.

so in my view tax invoice could not be issued

Dear Sir as per my knowledge Section 15 of Kvat Act dealer shall not be eligible for claim Output tax or set off input tax, but i want to know is there any other clarifcation for execution of work contract? Just becoz so many work contractor are giving tax invoice and claiming VAT which are under composition scheme. Some sand / stone chips suppliers also doing the same who is under COT.