Detroit's public sale of $275 million of bonds that financed the city's exit from bankruptcy has been delayed but should take place no later than early August, Reuters reported yesterday. Detroit is taking advantage of a new law that should give the bonds investment-grade ratings that could save the city between $20 million and $30 million over the life of the issue, according to the office of Michigan Governor Rick Snyder, a Republican. The law took effect in April and places a specific statutory lien on Detroit income tax revenue pledged to pay off the debt. The city is hoping the stronger payment pledge on the bonds will result in lower interest rates.