I thought it was interesting that the money works and decimal does not. I did a little more testing. Based on that testing, I would almost say that the ISNULL check implicitly converts the string to a float(or money) for the money data type for the comparison but does not do that for the decimal.

DECLARE @integer INT

,@Float FLOAT

,@Decimal DECIMAL(18,6)

,@Bit BIT

,@Money MONEY

,@String VARCHAR(20) = CAST('' AS FLOAT)

SELECT ISNULL(@Integer,@String),'Int'

SELECT ISNULL(@Float,@String),'Float'

SELECT ISNULL(@Decimal,@String),'Decimal'

SELECT ISNULL(@Bit,@String),'Bit'

SELECT ISNULL(@Money,@String),'Money'

It makes no sense that the decimal behaves this way when the money doesn't.