As a general rule, any coin that has been altered or modified from its original state is considered to be "damaged" and will not hold the same value as an unmodified coin. The extent to which the value is affected will depend on a number of factors, including the nature of the alteration, the condition of the coin, and the rarity of the issue.
In the case of a 1922 silver dollar with a letter stamped on it, the value of the coin will likely be reduced by a significant amount. This is because the modification has effectively damaged the coin and made it less desirable to collectors. The exact amount of the reduction in value will depend on the specific circumstances, but it is not uncommon for a modified coin to lose 50% or more of its potential value.
If you are considering purchasing a 1922 silver dollar with a letter stamped on it, it is important to do your research and understand the potential impact on the coin's value. You should also consider whether or not you are willing to pay a reduced price for a damaged coin.