@Omidmoj
You are correct that you are required to know your father‘s cost basis. Cost basis is determined by the US dollar value of the transaction. If the transaction is performed with a foreign currency, the US dollar value is determined using the currency exchange rate on the day of the transaction.
If your father is going to gift you tethered currency, your cost basis will be whatever he paid for it. You should get proof from your father, such as a copy of his receipt or online transaction documentation. You must understand that if you are audited, the IRS does not have to give you credit for any basis that you cannot prove with sufficient evidence. Even though tether is theoretically a fixed price of one dollar per unit, that is not always true, and the IRS could also take the position that without proof, your father might have received it as a gift himself, or received it for free in some other fashion and so you have no basis.
For example, suppose your father lives in France and buys US$100 of tether. That would cost him €88 today. If you were to be audited, you would need to show the transaction report confirming the purchase of the tether, and you would need to show the currency exchange rate for the day of the transaction, which together would show that your father‘s cost was US$100. (His basis also includes transaction costs. If your father paid a 3% commission, the cost basis would be $103.)
I don’t know what kind of documentation is provided with cryptocurrency transfers. You just have to be aware that if you sell cryptocurrency, and you are audited, the IRS does not have to award you any cost basis that you can’t prove.
