This post is pretty old but I ran into the same issue and resolved it with an expression.

=CDec(Left(Fields!Decimal.Value), 10)

Then in the Text Box Properties I would set it as a Number with 2 decimal places.

This way it would only show 2 decimal places in SSRS, but when you export it to excel it would retain the entire decimal value up to 10 decimals in my case. Adjust accordingly.:-D