pybill #2937 be able to make holdbacks [resolved]

Sometimes, you have to make holdbacks that will be invoiced six months later, when the system has proven it correctly works.

Today, you have to invoice a holdback with a negative price ... but something nicer could be done!

Warning : a holdback may not be submitted to VAT (the bill has all the amount of VAT but x% of the tax-free price is kept in an holdback, these x% will be invoiced later without VAT).

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