If you are considering offering full and final settlement to your creditors, the amount you are proposing to introduce will be on top of what you have already paid.
I'm not quite sure I understand the information you have provided here -
Original Debt: 17K
IVA: 13K
Paid Off: 3K
Monthly IVA: 230
I have assumed that you are 12 months in to your IVA and have to date paid £2,760, being 12 contributions of £230. You are proposing to introduce, for arguments sake, £8,000 from the sale of the asset (Which I asume is not yours, otherwise the sale proceeds would be introduced and the arrangement would also carry on until the end of year 5).
Introducing the £8,000 would mean total realisations of £10,760. That would mean a return to your creditrs of 63p in the £ before costs -
10,760 divided by £17,000 worth of creditors x 100 = pence in the £.
At this stage, if you are serious about proposing a variation to introduce the sum in full and final settlement, contact your IP and ask him / her to tell you what the return to creditors would be after costs. It is that figure that is most important. Then ask if they think that your creditors would accept this in full and final settlement of your debts.
Tell it like it is.