replace(/(d)(?=(ddd)+([^d]|$))/g, '$1 ')

each offer,index in offers_arr
    span.minsumm !{offer.mincreditsum}
    span.maxsumm !{offer.maxcreditsum}

script.
var myFunc = function (a) {
    return a.replace(/(d)(?=(ddd)+([^d]|$))/g, '$1 ');
}

each offer,index in offers_arr
 span#summin!= myFunc(offer.mincreditsum)