Tax override in SRM

Dear Team,

I have an issue in SRM purchase order where user says that Tax determined by external Tax system for particular case is not correct and tax needs to be manually overwritten,

The requirement is for US region and the Tax is calculated by external system - Taxware based on Ship to address Tax jurisdiction code. Based on Tax jurisdiction code the Tax calculated by Taxware is 6% but for the specific PO as per user the tax applicable is 1.875%. So business is asking for new custom field in SC/PO to enter manually the Tax rate and this tax rate should override the value proposed by external system.

So If any one has faced the same scenario and have proposed solution for it. please let me know your pointer on this

Thanks

Rajesh Rajagopal