SRM 7.0 : while editing the limit cart po not getting updated when budget increased

Hi All,

when editing the limit cart whenever we increase the amount PO is not getting updated with the increased amount but when we decrease the amount the PO is getting updated, we are using WBS element for assigning budget when I checked the cart  in BBP_PD getting message 'budget exceeded error'.plz refer screens.

For example: WBS element available budget =      200000

                      limit cart is created =                         150000 (Po created )

                      increased cart with 500 =                  150500 (Value not updated )

                      decreased cart with 200 =                  150300 ( Value updated)

the same process is working fine for free text cart .

Please help me on this.