Unable to partially match Purchase order, getting a Validation Error : There must be an invoice quantity if there was a quantity ordered

This is caused when the amount you are matching is less than 0.01 of the Quantity value of the Purchase order and it round to Zero.
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So for example if it is a quantity of 1 and a Value of £100000 then if you Invoice just £50 then this will be below 0.01 of the quantity.
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The way to resolve this is instead switch the quantity and the Unit cost when entering the purchase order.