According to your description, I understand that you want to enable external sharing for a specific user to a specific domain, right?
As per my knowledge, I'm afraid it is currently not supported in SharePoint. We can only limit external sharing to specific users (security groups) or several specific domains, but we can not restrict to a specific user and a specific domain.
I suggest you share your requirement in this feedback portal: https://feedbackportal.microsoft.com/feedback/forum/e3f04660-2c1c-ec11-b6e7-0022481f8472. Product team actively monitors feedback there and will take into consideration if the idea is valuable.
Thanks!
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