Manfrotto 055DXProB and adjusting legs opening/closing tension

Yesterday I bought a new tripod Manfrotto 055DXPro B. Everything looks great and perfect... but a little issue: When I'm trying to open the lgs of tripod, one of them is pretty loose (comparing with two other legs).

Is there any solution (or tension adjustment method) to tighten that particular leg a little bit more? Or have to send it back for service?

If you are talking about the tension on the legs at the joint with the flip lever, the little plastic tool included with the tripod will adjust that. It is probably snapped onto one of the legs and has an inset hexagonal shape. On the opposite side of the flip lever is a screw covered with a piece of rubber. Take that rubber cover off, then use the tool to tighten the screw. Be careful to get the tool all the way on and square to the screw or it will be easy to strip the plastic on the tool.

I presume the tension you are referring to is the force required to spread open the legs? There is an allen head screw at the top of each leg hinge, try tightening that.