give access of only one user records to another user.

Hi Everyone,

                   Need some suggestion on this scenario:

                 I have two users , A and B having different business units. and B has privileges for entity lead to create , delete ,assign ,append, append to ,read and write at  business unit level... and share privilege at organisation level. now I want to give all the access to records of  A user to B user except  delete and assign..Means B user cant delete and  assign the records of A user .So, How should I do this...

    If anyone have any suggestion ..please provide might be it helps!

 Thanks!