Can an American obtain a mortgage loan from Japan where the mortgage rates are much lower?
Only for property located in Japan.
No, unless the bank is licensed to to do business, and licensed to make loans, in your state; and it would have to comply with all relevant US and state lending laws.
Sure–if they will do the mortage for property 8000 miles away. Makes no difference where the money comes from–OK
Only for property located in Japan and with a japanese co-signer.
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No, you can’t.