> select workingday from workingdays
> where sortorder = (select result from dayofweek_select(:In_Begin))
> into :Temp;

Gerhardus,
I haven't got a clue whether this will work or not, but have you tried

select workingday from workingdays
join daysofweek_select(:In_Begin) on result = sortorder
into :Temp;

Set