No -- if you pay in the £6k as an overpayment, rather than a full and final it will just be absorbed by the IVA, which will continue, as now, until you have made the full number of agreed payments. You do not just "owe" the agreed amount, you still owe the full original debt, plus fees and possible statutory interest and the object of the IVA is to pay back as much of this as possible.
£6k sounds like a reasonable offer. This would have to be paid, if accepted by creditors, direct from the donors account and you should make it clear that it is only available for the purposes of a F&F only. If the offer is rejected the money will be retained by the donors and will not be available towards the IVA.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014