We use density functional theory to investigate how the presence of polymers influence the interactions between adsorbing or nonadsorbing flat surfaces, forming a slit geometry. The study is primarily made under "restricted equilibrium" conditions, i.e., below a threshold separation, the chains are allowed to equilibrate conformationally between the surfaces, but they are unable to diffuse into or out from the slit. This is believed to be a reasonable model for commonly encountered experimental situations, when the surfaces are large and adsorbing and the polymers are long. Using a recently presented density functional theory, we are able to compare predictions for chains of finite and infinite length. A major advantage of the infinite chain formulation is that it is a high resolution theory, and predictions from the finite length version will exactly approach the ones for infinite chains, when the degree of polymerization becomes large enough. Our study spans across a relatively large number of relevant system parameters, such as bulk concentration, surface adsorption strength, polymer length, and chain flexibility. The response in terms of the net surface interaction, to a change of a system parameter, is often nonmonotonic. We also make comparisons with predictions it full equilibrium, where the diffusion constraint has been relaxed. We give a specific example of how a diffusion-limited process can lead to hysteresis effects, similar to those observed in many surface force measurements. Finally, in a separate section of this work, we have included some direct experimental comparisons, demonstrating the relevance and applicability of restricted equilibrium conditions to "real world" scenarios. The computational power of the density functional approach is here highlighted, with calculations including semiflexible 20000-mers.