Is there a way to declare a public variable in SQL2005

I'm storing some formulas in a table to calculate provions, some of these formulas contains variables (i.e. @Var1, @var2, ...), I'm excuting these formulas in a stored procedure as follows

set @cmd='FormulaToExecute'
exec (@cmd)

System is generating an error against every variable used inm the formula expression (Must declare the scalar variable "@Var1")