That's not how write offs work. If you spent $100 on materials making a widget, and valued it at $1000, then somehow lost it (eg. stolen, non paying customer), your profit is -$100[1], not -$1000. Likewise, you can't claim a $1000 loss on that debt, only $100. The only way you can claim $1000 is if you previously recorded the $900 in interest as profit (and paid taxes on it).

[1]

    profit = revenue - cost
           = 0 - $100
           = -$100

Is it? You're holding a contract that stipulates you're owed 1000 dollars...

Normally, you'd sell that on for more than the original 100 dollars.