Doesn't it violate vendor's terms and conditions ?

When you create purchase order, you have entered the both delivery cost conditions, that means that is decided by vendor and you are agreed that vendor will be charged two delivery costs for the purchase order. Now if you decide to pay only one delivery cost, then purchase order has to modified, else you have data inconsistency in PO and goods receipt. It is not possible to change these details during goods receipt. But I have still one question, does it make sense to enter same condition type with two values in PO condition?

That is a poor business case as it is unknown how much the material costs, another vendor could potentially be cheaper.

Certainly the vendor knows within hours (maybe a few days) after the order from where he is going to delivery, as he needs to plan his activities as well. So your buyer could request this information in an order confirmation or latest with an advanced shipping notification, which gives him still time to adjust the PO before a receipt is done.

We have similar cases when we procure from global vendors, but this is not effecting the price, we get the agreed price no matter from where it is delivered. But for European trade reporting (INTRASTAT) we have to know the dispatch country, which means we need to add a partner role GS in the PO, and this also requires a change of the PO if it is not known at the time of order creation

MIGO does not allow to change a freight, that's just a fact, any change to that would be a modification.

I assume that your receivers were able to take such decision in the legacy system, most receivers I worked with around the globe would neither have the information to make such decision nor would they have the rights to change the purchase order.

Why is the buyer not able to make the right entry when he creates the PO?