Third-party payer and only % vAT paid yourself

The windscreen of my car was cracked due to stone chips. The (comprehensive) insurance paid the invoice directly to the repairer, but I had to pay 50% of the vAT myself (the % that I can claim as an expense).
However, Dexxter does not allow me to set the % professional expense to 0% and the % vAT to 50%.
Does anyone have any idea how I can process this correctly in Dexxter?