Add days to date (char column with decimal) in DB2

I have a table named INVOICE_ET which has column of below definition

INVDATE CHARACTER 10

result of select INVDATE from INVOICE_ET is as below

INVDATE
2021-07-31

In another table INVOICE_DY, I have a column named INV_DAYS, which has below definition

INV_DAYS  DECIMAL 3

result of select INV_DAYS from INVOICE_DY is as below

INV_DAYS 
88

----- Now my question is, I am trying to add INVOICE_DY.INV_DAYS field with INVOICE_ET.INVDATE field, but the char casting is not working, Please suggest how can I add these two columns. I am trying to get something like below

Select a.INVDATE, b.INV_DAYS, (a.INVDATE + b.INV_DAYS) as DUE_DATE 
from INVOICE_ET a 
join INVOICE_DY b 
On a.INVNUM = b.INVNUM //(edited this portion, because join is based on Invoice numbers)

Result should be 
INVDATE       INV_DAYS       DUE_DATE 
2021-07-31    88             2021-10-27

Any suggestions/ thoughts are welcome.