Do you mean that the Q3 and S3 should always be equal? How Q3 and S3 get populated? Do they contain manual entries?
Please note that there is a flaw in the custom validation i.e. if you copy any value and paste it over the cell with custom validation, it will overwrite the validation rule and the custom validation will stop working.
Q and S are both manually entered currency amounts that can be zero (Empty), contain a number or not. They are equal most of the time but do not have to be. All I want to do is flag a "Possible" data entry error when there is a value entered in "S" that does not match the value in "Q".
In that case, select S3 and apply the Custom Data Validation using the formula give below...
That will give an error if the value entered in S3 is not equal to the value in Q3.
And if you enter the value in S3 first and then in Q3 and want to make sure that the value entered in Q3 must be equal to the value exists in S3, apply a Custom Data Validation in Q3 using the formula...
If that takes care of your original question, please accept this post as an Accepted/Best response which will mark your question as Solved.