I actually thought of a solution myself, that gives me the decimals I wanted.
basically, I take the number i want, say 1,234,567,890.123 tale away a rounded to zero version, leaving 0.123
I then do as @Akhtar.shahbaz suggested using the text version of the numbered rounded to zero . and then to add in the decimals i make a text version of it, shortening it by the len-1 of the string.
A bit convoluted but it seems to work, and can work for any number of decimals. you just modify the first round function to be the number you want. And you need the Round of the two rounded functions, otherwise anaplan doesn't do the sum correctly