RERAPP on Debit and Credit condition of the same contract in the same perio

Hello Gurus

I have a scenario here. My client wants a contract that specifies the tenant as a customer , and also as a vendor.When i try to make a periodic posting run i am getting the following error:

Sales tax data do not define output tax.

Actually i have a debit condition and a credit condition assigned to the same contract, when i probe the error message by double clicking on it , this is the full error text:

Sales tax data do not define output tax

Message no. RECD078

Diagnosis

In Customizing of the contract type, the setting in the user/offerer field specifies whether this is a customer contract (company code is the offerer) or a vendor contract (company code is the user). Depending on this setting, the posting term expects output tax (customer contract) or input tax (vendor contract) as sales-tax data.

Procedure

Switch to maintenance of posting terms, and correct this error there.

how can i assign 2 different tax types one input and one output to the same customer contract.

Very much await your response

David