There are p heterogeneous objects to be assigned to n
competing agents (n > p) each with unit demand. It is
required to design a Groves mechanism for this assignment
problem satisfying weak budget balance, individual
rationality, and minimizing the budget imbalance. This calls
for designing an appropriate rebate function. Our main
result is an impossibility theorem which rules out linear
rebate functions with non-zero efficiency in heterogeneous
object assignment. Motivated by this theorem, we explore
two approaches to get around this impossibility. In the first
approach, we show that linear rebate functions with nonzero
efficiency are possible when the valuations for the
objects have some relationship. In the second approach,
we show that rebate functions with non-zero efficiency are
possible if linearity is relaxed.