In this paper, we investigated the (r, Q) inventory system under both all-units discount and a resource limitation which was considered as a soft constraint in the system cost function. It was assumed that the shortage of resource can be satisfied by renting extra re source and so a resource shortage cost that is proportional to the amount of rented resource is incurred. In the case in which the per unit resource usage is linearly dependent to the unit purchasing price, the problem was analyzed and based on derived properties of the cost function, an algorithm with a one-dimensional search procedure was presented to find an optimal policy of the problem via obtaining the partially optimal policies. To improve the efficiency of the presented algorithm, the sub-algorithm which obtains the partial optimal policies was modified and an improved version of the presented algorithm was presented. To evaluate the efficiency of the presented algorithms, they were compared experimentally. Numerical evaluations showed that the modified algorithm is superior to the basic one in most of scenarios of involved parameters. Also by the experimental results, the optimal system cost increases in the cases with small amount of available resource and large value of unit resource short age cost.