Are there tax implications when transferring funds internationally using card payments between offices?

If you are running the transaction via your Merchant Account, come tax season, how would you classify the transaction? Needless to say, be it your partner or a customer, it was incoming revenue for your business - and would most likely be treated as a Sale. I am sure you would already have taken into account that when you are 'sending' money, that would be treated as an expense.

A Director's loan into the company could be another way to having this money transferred (but then this would become either a short-term or long-term liability on the bank).

There usually would not be any high-fees if the money is being injected into the company has equity (to which your co-founder is investing into).