If possible, I would rip out the weed barrier so that the topsoil (such as it is) will have some contact with the subsoil. At the same time, I would remove at least have of the sand because it'll do your lawn no favors - during dry spells, you'll most likely wind up with a 12' x 12' area of brown grass. Unless you plan to maintain your lawn like it's a golf course, you'll always have a "bad spot" in the lawn. When you do add the topsoil, mix it thoroughly with the additional sand. Be sure to not add any additional organic matter when you do this! See here for the science behind that; here's a relevant quote from that article:
...the problem with this practice is that within 10 years (conservatively) the organic amendment will have decomposed; one is then left with the original soil, which will have subsided and compacted during this time. You can see evidence of this practice by looking at older residential lawns; the lawns slope away from sidewalks and driveways and are inches below grade of surrounding surfaces. There is no way to incorporate additional amendment into permanent landscapes without damaging root systems.
So, how do you get rid of the sand? Social media works well - a post like "Free sandbox sand!" should be all it takes.