I just tried it out it Nav 5.0 SP 1 and I have no Issues on Decimals. Which Country Version r u using? Try checking it in “Standard Navision” CRONUS and then compare the both.
You need to look at the rounding precision fields in conjuction with the decimal places fields. If you have a rounding precision set to 0.01 you will get the same error message.
I believe this was resolved by one of the related posts in this forum, or on mibuso, I cannot recal, but I do remember answering the same question in numerous different places so stopped [:D]